Formation of Diaphragm: Separating Peritoneal and Pericardial Cavities

00:01 We will now continue following how the spaces inside the embryo will separate to become the pericardial, pleural, and peritoneal cavities.

00:11 This is largely accomplished by formation of the diaphragm.

00:14 Now, recall early on that we have the intraembryonic coelom present in a bit of a horseshoe shaped appearance around the developing embryo.

00:24 But as the heart folds forward, it pulls that cavity, in this case, the early pericardial cavity with it.

00:31 But one thing to notice is that as that happens, it also pulls a large strip of mesoderm called the septum transversum with it.

00:40 And the septum transversum moves from a space a bit superior to the forming head to a spot just inferior to the developing heart and this is gonna start pinching portions of the intraembryonic coelom.

00:54 In particular, it's going to create small little canals connecting the pericardial cavity above to the peritoneal cavity below and for good reason, these are called pericardioperitoneal canals.

01:09 Now, there's gonna be a lot of words starting with the letter P in this talk.

01:12 So just do your best to follow along with each of them as they go.

01:15 The septum transversum is gonna form the early diaphragm and pinch that cavity but not completely obliterate it.

01:22 To get a complete separation, we're gonna have to further compress the pericardioperitoneal canals and that's accomplished by in folding of some mesoderm from the lateral body wall.

01:35 We'll get back to that in a moment but first, let's discuss the innervation of the diaphragm.

01:39 For those of you who know your anatomy, you'll know that the diaphragm is innervated by the phrenic nerve and the phrenic nerve comes from C3, C4, C5, keep the diaphragm alive.

01:49 The phrenic nerve initially innervates the diaphragm when it's very high up in the middle cervical region.

01:56 So that makes sense.

01:57 It's gonna have its innervation be nearby.

01:59 But as the body elongates moving into week eight, the body may elongate but the diaphragm is more or less stuck in place.

02:08 And as it gets pulled away from C3, C4, C5, the nerves have no choice but to stretch out and maintain their connection to the septum transversum so that by the time we get to week eight, that phrenic nerve has taken a very extended course down the anterior neck alongside the pericardium in the chest, and finally, to the diaphragm which it innervates and that's one reason that the phrenic nerve is so prone to injury both in trauma to the neck and any sort of tumor affecting the lungs very medially or the pericardium.

02:41 Now, let's return to the body wall and the cavity within it.

02:47 The pericardioperitoneal canals are going to be closed not by the septum transversum alone but because some folds of the lateral body wall grow in and fuse to the septum transversum.

02:59 These folds are called pleuroperitoneal folds.

03:03 So say this five times fast if you can.

03:06 Ready. The pericardioperitoneal canals are closed by the pleuroperitoneal folds and once that's accomplished, you functionally separated the thoracic cavity from the peritoneal cavity.

03:20 Now, just a quick hint, we do not yet have pleural cavities up in the chest.

03:25 We'll discuss those a little bit later but right now, let's turn our attention to the diaphragm itself.

03:31 The diaphragm is now keeping the abdominal organs separate from the thoracic organs.

03:36 And that's important because the abdominal organs are under a considerable amount of pressure as they develop.

03:41 So the diaphragm continues developing its muscular wall.

03:46 Initially, the muscular wall is not present in the septum transversum or the pleuroperitoneal folds but grows in as the body expands from the lateral wall.

03:58 So the little purple rim of tissue that you see in this image is actually going to turn into the muscle of the diaphragm.

04:05 Note that we have the mesentery containing the foregut as well as the inferior vena cava and a bit of the esophagus hanging out from the posterior body wall, coming in, and surrounded by the developing diaphragm.

04:19 We're now gonna look at the same structure a little bit further in development and from its inferior side.

04:25 So it's as though we're in the abdomen looking up at the diaphragm and we can see the central tendon.

04:31 That non-contractile connective tissue portion of the diaphragm, it's derived from the septum transversum and the muscles of the diaphragm that allow it to dome up and flatten out when it contracts are gonna be coming from the lateral body wall.

04:46 Note that it has to allow the inferior vena cava, the esophagus, and the aorta to pass through it if we're gonna have blood flow and movement of food into the stomach.

04:57 So the inferior vena cava pierces it at T8, the esophagus at T10, and the abdominal aorta at T12.

05:04 Those are the levels at which they're gonna pass through the diaphragm.

05:08 If the diaphragm were flat, it would all be at one level but since its domed, there's a bit of a stretch from T8 to T10, to T12 as that happens.

05:17 So things that can go wrong.

05:19 If one of the pleuroperitoneal folds fails to completely fuse to the septum transversum, you have a connection between your abdominal peritoneal cavity and your thoracic pericardial cavity.

05:33 Now, that in it of itself wouldn't be problematic but the problem arises because the pressure in the abdomen is very great.

05:41 During development, it's typical for part of our midgut to herniate out into our umbilical cord and then, gradually return as more space becomes available but because the gut is simply responding to pressure, if I've got a hole in my diaphragm, instead of going out the umbilicus, my gut will herniate up into my thorax and it will not return prior to birth and as it moves into the thorax, it's going to compress the lungs and the heart and that's gonna cause hypoplastic formation of the lungs.

06:12 They will likely be nonviable and because the lungs are not essential for life up until birth, these embryos and fetuses can go to term but suddenly die after delivery as they're unable to breathe due to abdominal contents compressing the heart.

06:29 This is almost exclusively found on the left side.

06:32 In this image, we can see a superior view of the diaphragm, the vertebra, and the back, the imprint of the heart in its pericardial sac are in front and we can see then on the left side, there's a defect in the diaphragm that allowed gut to herniate through and compress the lungs.

06:48 There may be defects on the right but they're largely hidden by the fact that you've got a massive liver right there preventing any tremendous movement of gut through the right side.

07:00 So this tends to occur primarily on the left side and can be detected by ultrasound during healthy screenings and if you're very lucky, you may be actually able to go in and correct this defect with a mesh prior to delivery.

07:15 Actual intrauterine surgery can be used to correct these defects, move the gut contents back into the abdomen, and allow the lungs to develop normally.

07:24 A similar but slightly different condition is called eventration of the diaphragm.

07:30 This occurs when a portion of the diaphragm is not properly innervated and it becomes very floppy.

07:36 In this case, the gut contents don't enter the thoracic cavity but they push that floppy area of the diaphragm up and effectively invade the thoracic space.

07:47 So a similar outcome even though there's still a thin layer of diaphragm present between the gut contents and the lungs.

07:55 Congenital eventration of the diaphragm occurs almost exclusively on the left. The treatment is usually after delivery. Most patients are operated during the neonatal period.

08:05 The prognosis is generally better as compared to congenital diaphragmatic hernia.

08:10 Thank you very much for your attention and I'll see you at our next talk.